This invention relates to the production of tubular conductors which consist of a niobium layer and a copper layer, and more particularly, to an improved method for making such conductors, which conductors are particularly useful in superconducting cables.
It is well known that niobium is eminently suitable as a superconductor material for superconducting cables which are used for transmitting large amounts of electrical energy. In particular, its application for superconducting single- and three-phase cables is known. Niobium has a very high lower critical magnetic field H.sub.C1 of about 120,000 A/m and relatively low a-c losses as long as this critical magnetic field is not exceeded. In superconducting cables, niobium can be used to advantage in the form of a thin layer applied to a tubular carrier of a metal such as copper, which at the temperature required for maintaining superconductivity of the niobium, i.e., about 4 to 5 K, is electrically highly normal-conducting and has a high thermal conductivity. For superconducting single- or three-phase a-c cables, a coaxial arrangement of such copper tubes, each provided with a niobium layer externally or internally would seem to be particularly advantageous. Preferably, a niobium layer is provided on the outside of the inner tube and on the inside of the outer tube of a pair of coaxial conductors. By using the inner tube as the outgoing conductor and the outer tube as the return conductor, electric and magnetic fields will be maintained only in the space between the niobium layers with the copper tubes remaining free of fields, so that no eddy current losses will occur therein. In the operation of such cables, a cooling medium such as liquid helium, will be supplied so that it flows along, particularly within the tubular inner conductor and over the outside of the tubular outer conductor. In this manner, the cooling medium is in direct contact with the copper surface of each tube (see "Elektrotechnische Zeitschrift -- Edition A," Vol. 92 (1971), p. 740 to 745.)
In tubular conductors of this nature having a niobium and a copper layer, the copper is used for electrical stabilization of the superconducting niobium in that, if the niobium changes over from superconducting to the electrically normal-conducting state, which can occur in the event of an overload, the copper is capable of carrying at least part of the current flowing in the superconducting niobium, and of transferring to the contiguous cooling medium the heat loss produced in the niobium or resulting from a-c losses. For this stabilization to be effective, the best possible electrical and thermal contact between the niobium layer and the copper is necessary. Additionally in such cables, the surface of the niobium layer should be as smooth and free of disturbances as possible, since the a-c losses occuring in the niobium in the superconducting state increase relatively steeply with increasing surface roughness of the niobium. Because when operating below the critical field intensity H.sub.C1, the current flowing in the superconducting niobium layer flows only in a thin surface layer, which is generally less than about 0.1 .mu.m, the niobium layer can be made relatively thin, for example, with a thickness of between about 0.1 and 0.0l mm. By doing so, a large savings of expensive material is obtained.
The manufacture of tubular conductors of this type which have a niobium and a copper layer, involves great difficulties, since a good electrical and thermal contact between niobium and copper is very difficult to obtain and furthermore, because the mechanical union between niobium and copper breaks easily, for example, in the event of deformation causing the niobium layer to chip off the copper layer. The good surface quality is referred to above, in order to keep a-c losses at a minimum, also are difficult to obtain.
A method of depositing relatively pure and well-adhering niobium layers on a suitable carrier such as copper has been disclosed in an article by Mellors and Senderoff in "Journal of the Electrochemical Society," Vol. 112 (1965) p. 266 to 272. In the method disclosed therein, the niobium layer is deposited by fusion electrolysis from a melt consisting mainly of alkali fluorides and a niobium fluoride. However, the application of this method for the deposition of niobium onto copper tubes of great length such as those required for cables in order to avoid unnecessary welded joints, involves great difficulties. The tubes to be plated would have to be introduced into a molten electrolyte having a temperature of at least approximately 740.degree. C. through a vacuum-tight air lock. Furthermore, in a fusion-electrolysis plating process, the application of a niobium layer to the inside of a long copper tube would involve additional problems because of insufficient accessibility.
A method of manufacturing tubular conductors having a niobium layer and a copper layer has been disclosed in U.S. Pat. No. 3,777,368 granted Dec. 11, 1973 and assigned to the same assignee as the present invention. In the method disclosed therein, a strip consisting of a niobium layer and a copper layer, and having niobium flanges at its edges is first produced, then bent to form a tube with the niobium flanges abutting and the niobium flanges then joined together by electron beam welding. While this method, which furnishes a tube having a welded seam is suitable for producing tubes for superconducting cables having a niobium and a copper layer, it is still relatively costly, particularly because of the bending and welding operations required.
Thus, it can be seen that there is a need for an improved and simplified method of manufacturing tubular conductors of this nature which consist of a niobium and copper layer, and in particular, such a method which permits manufacturing seamless tubes having good contact between the niobium and copper layers.